1. Field of the Invention
The present invention relates to a method for forming an isolation in a semiconductor device, and more particularly to increasing the active region in higher integrated circuits by diminishing the bird's beak.
2. Description of the Prior Art
Generally, a field oxide layer in integrated circuits has been commonly formed by using the LOCOS(local oxidation of silicon) process. However, the LOCOS process, which involves increasing the size of the bird's beak, causes a decrease of the active regions in semiconductor devices.
A conventional method for forming a field oxide layer will be described below in detail, referring FIG. 1.
First, FIG. 1 shows the PBL(polysilicon buffered LOCOS) process which can more diminish the bird's beak.
As shown in FIG. 1, a pad oxide layer 12 is formed on a silicon substrate 11. Also, a polysilicon layer 13, a nitride layer 14 and a photoresist layer (not shown) are in order formed on the pad oxide layer 12. After defining a field region using a predetermined photomask, the portions of the nitride layer 14 and the polysilicon layer 13 are etched such that a portion of the silicon substrate 11 is exposed, and a field oxide layer 15 is formed in the exposed silicon substrate 11 by the thermal oxidation process.
However, when the thermal oxidation process is applied to the exposed silicon substrate 11 so as to form the field oxide layer, a bird's beak is formed along the interface between the polysilicon layer 13 and the silicon substrate 11 in spite of the polysilicon layer's consumption. Since the increase of the size of the bird's beak causes the diminution of the active region of the semiconductor device, it is very difficult to highly integrate the devices.